Oxygen-Producing Inert Anodes for Som Process

ABSTRACT

An electrolysis system for generating a metal and molecular oxygen includes a container for receiving a metal oxide containing a metallic species to be extracted, a cathode positioned to contact a metal oxide housed within the container; an oxygen-ion-conducting membrane positioned to contact a metal oxide housed within the container; an anode in contact with the oxygen-ion-conducting membrane and spaced apart from a metal oxide housed within the container, said anode selected from the group consisting of liquid metal silver, oxygen stable electronic oxides, oxygen stable crucible cermets, and stabilized zirconia composites with oxygen stable electronic oxides.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government Support under Contract NumberDE-FC36-04GO14011 awarded by the Department of Energy. The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

Electrolysis is a common form of electrochemical refining. In anelectrolysis process, the ore is dissolved in an aqueous or non-aqueoussolution or melted in an electrolytic furnace. Once dissolved or melted,the ore dissociates into ionic species, forming an electrolyte. Themetallic components of the ore to be extracted become positively chargedcations. The remaining components, typically oxygen and halogens, becomenegatively charged anions. To extract the metal from the ore, anelectric potential is applied across two electrodes that are immersed inthe electrolyte. The metallic ions are thereby attracted to thenegatively charged cathode, where they combine with electrons and aredeposited as metal. The counter ions, most typically oxygen andhalogens, are driven to the positively charged anode and evolve as wastegases.

Oxygen-ion-conducting membranes, such as yttria-stabilized zirconia,have been used in electrolytic metal extraction processes and permit theextraction of pure metals from their respective oxides. A metal oxidedissolved in a suitable molten electrolyte is separated from the anodeby oxygen-ion-conducting membrane. When a potential is generated betweenthe cathode (in contact with the electrolyte) and the anode (in contactwith the oxygen-ion-conducting membrane), and the dissociation potentialof the oxides are exceeded, the oxygen species of the electrolyte istransported across the ion-conducting membrane and is oxidized at theanode, and the metallic species are reduced at the cathode. Thiselectrochemical cell is referred to as a solid oxide membrane (SOM)cell.

The oxidizing environment of the anode limits the available choices foran anode. In order to reduce the oxidizing environment of the anode, aconsumable carbon source or consumable reducing gas such as carbonmonoxide (CO) and/or hydrogen is typically continuously fed over theanode to getter or react with the oxygen generated at the anode. Thisreduces the corrosive oxidative environment at the anode; however,reducing gases and in particular hydrogen are expensive and can addsignificantly to the cost of metal extraction. Due to the amount ofreductant gas required, H₂ gas has to be fed in continuously. This putstechnique challenges on hydrogen transportation, storage and safety.

An electrolysis cell useful in the synthesis of metals from theirrespective oxides is desired; in particular, anodes that are stableunder oxidizing conditions of the anode are desired. An electrolysissystem that does not require a reductant is desired.

SUMMARY OF THE INVENTION

The present invention describes an oxygen producing inert anode. Theanode eliminates the need to use hydrogen or CO as getters to react withoxygen. This results in a “greener” SOM process as it can be driven byelectricity alone.

In one aspect of the invention, an electrolysis system for generating ametal and molecular oxygen is provided that includes a container forreceiving a metal oxide containing a metallic species to be extracted, acathode positioned to contact a metal oxide housed within the container;an oxygen-ion-conducting membrane positioned to contact a metal oxidehoused within the container; an anode in contact with theoxygen-ion-conducting membrane and spaced apart from a metal oxidehoused within the container, said anode selected from the groupconsisting of liquid metal silver or its alloys (Silver-Copper,Silver-Tin etc.), oxygen stable electronic oxides, oxygen stablecermets, and stabilized zirconia composites with oxygen stableelectronic oxides.

In another aspect of the invention, a method of metal extraction isprovided that includes (a) providing a cell comprising a metaloxide-containing electrolyte comprising a metallic species to beextracted, said electrolyte in communication with a cathode and anoxygen-ion-conducting membrane; and an anode in communication with theoxygen-ion-conducting membrane, said anode selected from the groupconsisting of liquid metal silver or its alloys (Silver-Copper,Silver-Tin etc.), oxygen stable electronic oxides, oxygen stablecermets, and stabilized zirconia composites with oxygen stableelectronic oxides; and (b) applying a potential across the cathode andanode that is greater than the dissociation potential of the metaloxide, wherein the metallic species are reduced at the cathode and theoxygen species are oxidized at the anode to form molecular oxygen.

BRIEF DESCRIPTION OF THE FIGURES

The foregoing and various other aspects, features, and advantages of thepresent invention, as well as the invention itself, may be more fullyappreciated with reference to the following detailed description of theinvention when considered in connection with the following drawings. Thedrawings are presented for the purpose of illustration only and are notintended to be limiting of the invention, in which:

FIG. 1 is a conceptual representation of the solid oxide membrane cellwith oxygen producing liquid metal anodes used for electrolyzingexemplary metal oxides (MgO).

FIG. 2A is a schematic illustration of the reactor employing ayttria-stabilized zirconia (YSZ) tube as part of the solid oxidemembrane cell with an oxygen-generating liquid metal anode forelectrolyzing MgO; and FIG. 2B is a cross-sectional view of the reactoracross line A-A.

FIG. 3 is a plot of potential (and current density) vs. time duringoperation of the reactor according to one or more embodiments of thecurrent invention.

FIG. 4 is a plot of current density vs. applied potential for an MgOelectrolysis at 1150° C. showing the dissociation potential of MgOwithout the use of reductants.

FIG. 5 is a plot of current density vs. applied potential for an MgOelectrolysis at 1150° C. showing the dissociation potential of MgO withhydrogen or carbon as a reductant.

DETAILED DESCRIPTION OF THE INVENTION

An environmentally sound solid-oxide-membrane (SOM) electrolysis systemcan efficiently synthesize metals and alloys directly from their oxideores with minimum feed-material preparation and produce oxygen gas orwater vapor as the major byproduct. In particular, high-energy-contentmetals, e.g., metal oxides having large dissociation energies such asmagnesium, tantalum and titanium, are synthesized directly from theirrespective oxides (dissolved in fluoride-based molten fluxes) byemploying oxygen-ion-conducting solid electrolyte with anoxygen-producing anode and an inert current collector. Duringelectrolysis, metal cations are reduced at the cathode and molecularoxygen gas is generated at the anode. No gettering gas is required andthe oxygen is collected at the anode.

Exemplary oxygen-producing anodes include liquid silver (Ag) or itsalloys (Silver-Copper, Silver-Tin etc.), cermets, electronic oxides andcomposites thereof with stabilized zirconia. The oxygen-producing anodeis stable under the oxidizing and high temperature (e.g., >1000° C.)conditions of the anode. Suitable oxygen-producing anodes possess highelectronic conductivity (>10 S/cm) and charge transfer/surface exchangekinetics (>10-7 cm/s), are stable in the anodic gas environments and arechemically, mechanically and structurally compatible with theoxygen-ion-conducting membrane.

In one or more embodiments, the oxygen-producing anode is liquid silverand no gettering agent, e.g., H₂/CO, is required. The oxygen enteringthe liquid silver anode through the oxygen-ion-conducting membraneevolves as oxygen gas since silver oxide is not stable at the operatingtemperature (1100-1300° C.).

In other embodiments, the oxygen producing inert anodes is a porouscermet. The cermet is a composite of an oxygen stable (noble) metal suchas iridium or platinum and a stabilized zirconia. The noble metal iscapable of withstanding the high temperatures during operation withoutmelting. Stabilized zirconia may be a rare earth element- or alkalineearth-stabilized zirconia, such as zirconia stabilized with yttria,calcium oxide, scandium oxide and the like. The cermet anode istypically coated as a porous thin film on a support, which can be theoxygen-ion-conducting membrane. An exemplary cermet anode is about20-40% porous to permit molecular diffusion of gases.

In other embodiments, the anode can be an oxygen stable electronic oxidesuch as strontium-doped lanthanum manganite (La_(1−x)Sr_(x)MnO₃ or LSM).Still other stable electronic oxides include A-site deficientacceptor-doped lanthanum ferrite and lanthanum cobaltite, e.g.,La_(1−x)A_(x)FeO₃ or La_(1−x)A_(x)CoO₃. The anode materials may includeone or more dopants from the group consisting of Ca, Ce, Pr, Nd, and Gdin the La site and from the group consisting of Ni, Cr, Mg, Al, and Mnin the Fe or Co site; Fe may also be used as a dopant in the cobaltsite. The electronic oxide is typically coated as a porous thin film ona support, which can be the oxygen-ion-conducting membrane. An exemplaryelectronic oxide is about 20-40% porous to permit molecular diffusion ofgases.

In still other embodiments, the oxygen-producing anode may be acomposite anode including an electronic oxide as described above and astabilize zirconia as described above. The composite anode is typicallycoated as a porous thin film on a support, which can be theoxygen-ion-conducting membrane. An exemplary anode is about 20-40%porous to permit molecular diffusion of gases.

The dopant materials and amounts for electronic oxides such as lanthanumferrite and the lanthanum cobaltite may be selected to decrease thethermal expansion of the ceramic and to provide a closer thermal matchto the stabilized zirconia.La(Ca,Ce,Sm,Pr,Gd,Nd)Fe(Mn,Ni,Al,Cr,Mg)O_(3−x) andLa(Ca,Ce,Sm,Pr,Gd,Nd)Co(Mn,Ni,Al,Cr,Mg,Fe)O_(3−x) powders of varyingcompositions can synthesized by mixing high purity precursors ofcarbonates and oxides in appropriate stoichiometric ratios and calciningthem at a temperature of 1200-1300° C. for 4 hours in air. The calcinedpowders can be lightly crushed using an alumina mortar and pestle andthe calcination step can be repeated to complete the solid-statereaction. The lanthanum ferrite and lanthanum cobaltite cathodematerials systems may be fabricated on the appropriate support materialand are typically prepared on the oxygen-ion-conducting membrane. In oneor more embodiments, the porous anode is supported on a YSZ membrane.

Suitable solid oxide electrolytes for use as the oxygen-ion-conductingmembrane are solid solutions (i.e., solid “electrolytes”) formed betweenoxides containing divalent and trivalent cations such as alkaline earthoxides, e.g., calcium oxide, or rare earth oxides, e.g., scandium oxide,yttrium oxide, lanthanum oxide, etc., and oxides containing tetravalentcations such as zirconia, hafnia, thoria and ceria. The oxygenion-conducting materials or phases may be an oxygen-ion- conductivemixed metal oxide having a fluorite structure. The oxygen ion conductingmaterial may be a doped fluorite compound. The higher ionic conductivityis believed to be due to the existence of oxygen ion site vacancies. Oneoxygen ion vacancy occurs for each divalent or each two trivalentcations that are substituted for a tetravalent ion in the lattice.

Any of a large number of oxides such as rare earth doped zirconia-,ceria-, hafnia-, or thoria-based materials may be used as the solidoxide electrolyte. Some of the known solid oxide materials include rareearth or alkaline earth-doped zirconia-, ceria-, hafnia-, andthoria-based oxides, such as Y₂O₃-stabilized ZrO₂ (YSZ), CaO-stabilizedZrO₂, Sc₂O₃-stabilized ZrO₂, Y₂O₃-stabilized CeO₂, CaO-stabilized CeO,GaO-stabilized CeO₂, ThO₂, Y₂O₃-stabilized ThO₂, or ThO₂, ZrO₂, CeO₂, orHfO₂ stabilized by addition of any one of the lanthanide oxides or CaO.Additional examples include strontium- and magnesium-doped lanthanumgallate (LSGM). Many other oxides are known which have demonstratedoxygen ion-conducting ability, which could be used as theoxygen-ion-conducting membrane. The solid oxide electrolyte membrane canbe in any shape. One particularly convenient shape is tubular, with oneend of the tube being closed. Another suitable shape is in the form of aflat sheet or incorporated into a container for holding the molten metalflux.

The system and method according to one or more embodiments of thepresent invention may be use to extract high energy content metals, suchas magnesium. The current production methods for magnesium are eithermetallothermic reduction (magnetherm process) at high temperatures(1,600° C.) involving expensive metal reductant (FeSi) or electrolysisfrom a halide electrolyte bath that requires extensive and expensivefeed-material preparation. Both these techniques are also energyintensive, have low yield and generate large quantities of wastereaction products harmful to the environment. In the SOM process, theoxide reduction is electrochemical and has efficiencies close to 100%.Unlike the current metallothermic and the electrolytic processes, theSOM is more economic and less energy intensive, and its process productsare environmentally benign.

FIG. 1 shows the SOM cell configuration for synthesizing Mg from MgOwith oxygen producing liquid silver anode. The experimental cell withliquid silver anode can be described as:

-   -   Ag (l)/Yttria Stabilized Zirconia (YSZ)/ionic flux with        dissolved MgO/Steel.

The individual half-cell reactions can be written as follows:

-   -   At the cathode: Mg₂₊+2e⁻→Mg(g)    -   At the flux/YSZ interface: O²⁻(flux)=O²⁻(YSZ)    -   At YSZ/liquid anode interface: O²⁻(YSZ)=[O]_(Ag anode)+2e⁻ and        [O]_(Ag anode)=½O₂(g)    -   Overall cell reaction can be given as: Mg²⁺+O²⁻=Mg(g) +O₂(g)

In this process the oxygen-ion-conducting membrane, shown here as YSZ,separates the inert cathode and the flux from the liquid metal anode orthe oxygen-producing anode. The Mg-containing flux has high ionicconductivity, high oxide solubility and low viscosity. In the exemplarysystem of FIG. 1, the dissolved oxide in the flux is MgO. When theapplied electrical potential between the electrodes exceeds thedissociation potential of the oxide, oxygen ions are pumped out of theflux and through the YSZ membrane to the anode. Mg(g) evolves at theinert cathode (steel) that is condensed in a separate chamber yielding ahigh-purity Mg metal. If liquid anode is used, the oxygen dissolves inthe liquid anode, [O]Anode, and evolves as oxygen gas O₂(g).

Liquid silver (Ag (l)) electrode serves as a medium to carry out thecharge-transfer reaction involving oxygen ions and soluble oxygen at theYSZ/silver interface followed by oxygen gas evolution as shown in theabove figure. Liquid metal anodes such as silver have low vaporpressure, high oxygen solubility and high oxygen diffusivity in thetemperature range of interest. Other oxygen-producing anodes that arestable under the oxidizing conditions of the anode may be used. Suchanodes include cermets, electronic oxides such as strontium-dopedlanthanum manganite (La_(1−x)Sr_(x)MnO₃ or LSM), acceptor-dopedlanthanum ferrite and lanthanum cobaltite materials, e.g.,La_(1−x)A_(x)FeO₃ or La_(1−x)A_(x)FeO₃, and composites thereof withstabilized zirconia.

The anodic and cathodic reactions and the transport of various speciesare as shown in FIG. 1. The rate of the slowest step determines theoverall metal production rate in the cell. In order to increase theoverall rate, the rate of the slowest step needs to be enhanced. Theflux is an electron blocker and ionic resistance of the flux is muchsmaller than that of the YSZ membrane. Adequate stirring of the flux andhaving sufficient MgO in the flux are help to ensure that transport inthe flux is rapid. The temperature is sufficiently high (≧1000° C.) socharge transfer reactions are rapid. Since the oxygen solubility anddiffusivity are high in the liquid anode and the anode is well stirredby the evolving O₂(g), oxygen transport in the liquid anode is alsorapid. The free energy change of oxygen evolution under appliedpotential at these temperatures indicates that the oxygen evolutionoccurs readily.

The production of magnesium without reductant gases is described.Magnesium is produced by SOM process without any reductant feeding andit is expected that oxygen generated at the anode can be separated fromthe exhaust gas flow for other industrial applications. With referenceto FIG. 2A, an exemplary electrolytic cell and magnesium collectionapparatus is designed to produce and contain 100-200 g of magnesiummetal. The electrolytic cell shown in cross-section along line A-A inFIG. 2B. The electrolyte cell can utilize up to 33 cm² of the liquidanode area and operate at anodic current densities as high as 1 A/cm².The YSZ solid electrolyte is in the form of a one-end-closed tube (1.9cm OD, 1.42 cm ID, 20 cm long) that contains the liquid anode.Experiments were conducted employing liquid silver as the anode. Due tosilver's high oxygen solubility, diffusivity and wetting of the YSZmembrane, it was used inside the YSZ membrane, as a connection betweenYSZ membrane and iridium lead wire (current collector). Other noblemetals such as platinum or a solid (non-porous) sintered rod of theaforementioned anode material (electronic oxides, cermets and electronicoxide composites) may also be used as current collectors. The steelcrucible that holds the MgO containing ionic flux served as the cathode.In order to protect the YSZ tube above the flux from the Mg vapor thatwas produced along the wall of the stainless steel container (cathode),argon gas was introduced into the chamber as a carrier gas and diluent.The argon-magnesium gas mixture passed out of the electrolysis chamberto the lower condensation chamber (not shown) where the Mg(g) wascondensed.

During operation, a DC voltage greater than the dissociation potentialof MgO was applied. Mg²⁺ cations moved toward the cathode, gainedelectrons and were reduced to Mg. At the experimental temperature 1150°C., Mg evolved as Mg gas. O²⁻ anions in the slag, driven byelectrochemical potential difference, passed through the YSZ membrane,which is an oxygen ion conductor, toward the anode. At the interface ofmembrane and silver, O²⁻ lost electrons, and associated with each otherto form O₂ gas and the O₂ gas was carried away by input Ar gas flow.

As shown in FIG. 3, during the SOM experiment, a DC voltage was appliedbetween anode and cathode, the voltage was initial increased linearlyfrom 0 volt to 6 volt and kept at 6 volt for 6.5 hours. FIG. 3 alsoshows, through the current density versus time curve, that the cellbecame stable in about three hours and after one and half hours, thecurrent density started decreasing.

After experiment, the stainless thin foil with the collected magnesiumwas pulled out from the condenser and the total weight was measured.Compared with the weight of the stainless steel sheet without magnesiumcondensation, it turned out that 8.1 grams of magnesium was produced.EDAX analysis shows the product is pure magnesium.

The cell was characterized using impedance spectroscopy,potentio-dynamic sweeps and potentio-static holds. The electrochemicalinstrumentation consists of a Princeton Applied Research (PAR)potentiostat (Model 263 A) and Solartron impedance analyzer (Model 1250B). A KEPCO® power booster was used to increase the current limit of thepotentiostat to 10 amps. Data acquisition and control of the aboveinstruments was achieved with CorrWare® and Zplot® (software) fromScribner Associates (Southern Pines, N.C.). A Hewlett Packard powersupply (Model 6033A) was used to apply a constant potential to the cellfor electrolysis. The applied electrical potential and resulting currentfrom the cell were logged at 1 second intervals using a Fluke Hydra®data logger (Model 2635A).

The MgO dissociation potential measurement for the above cell wasdetermined based on the setup response to the slow potenio-dynamic sweep(scan rate=0.5 mV/sec) across the cell, as is shown in FIG. 4. FIG. 4shows the results of potentio-dynamic sweep and indicates that thedissociation potential of MgO in the experiment is about 1.15V, which ishigher than the dissociation potentials when either carbon or hydrogenis used as reductant (shown in FIG. 5). Comparison between FIG. 4 andFIG. 5 shows that to obtain the same current density, higher voltage hasto be applied in the SOM experiment without reductant than in the SOMexperiment with either carbon or hydrogen as reductant. This suggeststhat the to get the same amount of magnesium, more electrolyte power (5%more when applied potential is 6V) has to be used in an oxygen-producingsystem, but this can be justified by the savings in the carbon orhydrogen feeding during magnesium production, and the benefit of oxygenobtained.

1. A method of metal extraction comprising: (a) providing a cellcomprising: a metal oxide-containing electrolyte comprising a metallicspecies to be extracted, said electrolyte in communication with acathode and an oxygen-ion-conducting membrane; an anode in communicationwith the oxygen-ion-conducting membrane, said anode selected from thegroup consisting of liquid metal silver or its alloys (Silver-Copper,Silver-Tin ) oxygen stable electronic oxides, oxygen stable cermets, andstabilized zirconia composites with oxygen stable electronic oxides; and(b) applying a potential across the cathode and anode that is greaterthan the dissociation potential of the metal oxide, wherein the metallicspecies is reduced at the cathode and the oxygen species is oxidized atthe anode to form molecular oxygen.
 2. The method of claim 1, whereinthe anode comprises liquid silver or its alloys (Silver-Copper,Silver-Tin).
 3. The method of claim 1, wherein the anode is in the formof a thin film.
 4. The method of claim 3, wherein the anode is porous.5. The method of claim 4, wherein the anode is about 20% to about 40%porous.
 6. The method of claim 3, wherein the thin film anode ispositioned on the oxygen-ion-conducting membrane.
 7. The method of claim1, wherein the anode comprises an electronic oxide selected from thegroup consisting of strontium-doped lanthanum manganite, acceptor-dopedlanthanum ferrite and acceptor-doped lanthanum cobaltite.
 8. The methodof claim 1, wherein the anode is selected from the group consisting ofLa(Ca,Ce,Sm,Pr,Gd,Nd)Fe(Mn,Ni,Al,Cr,Mg)O_(3−x) andLa(Ca,Ce,Sm,Pr,Gd,Nd)Co-(Mn,Ni,Al,Cr,Mg,Fe)O_(3−x).
 9. The method ofclaim 1, wherein the anode comprises a cermet composite comprising anoble metal having a melting point above the operating temperature ofthe cell and stabilized zirconia.
 10. The method of claim 1, wherein themetal is selected from the group consisting of magnesium, tantalum andtitanium.
 11. The method of claim 1, wherein the oxygen ion-conductingmembrane is selected from the group consisting of rare earth or alkalineearth-doped zirconia-, ceria-, hafnia-, and thoria-based oxides.
 12. Themethod of claim 7, wherein the membrane comprises yttria-stabilizedzirconia.
 13. The method of claim 1, wherein the cell is maintained atemperature greater than about 1000° C.
 14. The method of claim 1,wherein the cell is maintained at a temperature in the range of about1000° C. to about 1300° C.
 15. The method of claim 1, further comprisingcollecting molecular oxygen at the anode.
 16. An electrolysis system forgenerating a metal and molecular oxygen, comprising: a container forreceiving a metal oxide containing a metallic species to be extracted, acathode positioned to contact a metal oxide housed within the container;an oxygen-ion-conducting membrane positioned to contact a metal oxidehoused within the container; an anode in contact with theoxygen-ion-conducting membrane and spaced apart from a metal oxidehoused within the container, said anode selected from the groupconsisting of liquid metal silver or its alloys (Silver-Copper,Silver-Tin), oxygen stable electronic oxides, oxygen stable cermets, andstabilized zirconia composites with oxygen stable electronic oxides 17.The electrolysis system of claim 16, wherein the oxygen ion-conductingmembrane is selected from the group consisting of rare earth dopedzirconia-, ceria-, hafnia-, and thoria-based oxides.
 18. Theelectrolysis system of claim 16, wherein the membrane comprisesyttria-stabilized zirconia.
 19. The electrolysis system of claim 16,wherein the anode is liquid silver or its alloys (Silver-Copper,Silver-Tin).
 20. The electrolysis system of claim 16, wherein the anodeis in the form of a thin film.
 21. The electrolysis system of claim 16,wherein the anode is porous.
 22. The electrolysis system of claim 16,wherein the anode is about 20% to about 40% porous.
 23. The electrolysissystem of claim 20, wherein the thin film anode is positioned on theoxygen-ion-conducting membrane.
 24. The electrolysis system of claim 16,wherein the anode comprises an electronic oxide selected from the groupconsisting of strontium-doped lanthanum manganite, acceptor-dopedlanthanum ferrite and acceptor-doped lanthanum cobaltite.
 25. Theelectrolysis system of claim 16, wherein the anode is selected from thegroup consisting of La(Ca,Ce,Sm,Pr,Gd,Nd)Fe(Mn,Ni,Al,Cr,Mg)O_(3−x) andLa(Ca,Ce,Sm,Pr,Gd,Nd)Co-(Mn,Ni,Al,Cr,Mg,Fe)O_(3−x).
 26. The electrolysissystem of claim 16, wherein the anode comprises a cermet comprising acomposite comprising noble metal having a melting point above theoperating temperature of the cell and stabilized zirconia.
 27. Theelectrolysis system of claim 16, wherein the metal is selected from thegroup consisting of magnesium, tantalum and titanium.